Tuning the Driving Force for Charge Transfer in Perovskite–Chromophore Systems

Understanding the interplay between the kinetics and energetics of photophysical processes in perovskite–chromophore hybrid systems is crucial for realizing their potential in optoelectronics, photocatalysis, and light-harvesting applications. By combining steady-state optical characterizations and transient absorption spectroscopy, we have investigated the mechanism of interfacial charge transfer (CT) between colloidal CsPbBr3 nanoplatelets (NPLs) and surface-anchored perylene derivatives and have explored the possibility of controlling the CT rate by tuning the driving force. The CT driving force was tuned systematically by attaching acceptors with different electron affinities and by varying the bandgap of NPLs via thickness-controlled quantum confinement. Our data show that the charge-separated state is formed by selectively exciting either the electron donors or acceptors in the same system. Upon exciting attached acceptors, hole transfer from perylene derivatives to CsPbBr3 NPLs takes place on a picosecond time scale, showing an energetic behavior in line with the Marcus normal regime. Interestingly, such energetic behavior is absent upon exciting the electron donor, suggesting that the dominant CT mechanism is energy transfer followed by ultrafast hole transfer. Our findings not only elucidate the photophysics of perovskite–molecule systems but also provide guidelines for tailoring such hybrid systems for specific applications.

Compound 2 (0.50 g, 0.61 mmol) was dissolved in DCM (10 mL) in a round-bottom flask (50 ml). Trifluoroacetic acid (3 mL) was added to this solution. The combined reaction mixture was stirred for 1 h at room temperature. The progress of the reaction was thoroughly followed by TLC analysis of removed aliquots (50:1 DCM-EtOH). After complete consumption of the starting material, more DCM (50 mL) was added. The resultant solution was washed first with aqueous K2CO3 and then with water. The organic phase was collected and concentrated. The crude product was then chromatographed on silica with 20:1 DCM-EtOH mixture to yield the pure product (0.43 g, 98%) as an orange powder. 1    Note that a small amount of ferrocene is added to these measurements (the small signal at 0V) as internal standard/calibration    Note that the probe light for measuring PMIDE was generated using a sapphire crystal.

Estimation of the exciton binding energy and difference in CT driving force
To have an approximate estimation of the exciton binding energy of NPLs, their absorption spectra in hexane were normalized and fitted with a quantum-well absorption model 4,5 , which has been widely used for extracting exciton binding energy in CsPbBr3 NPLs [6][7][8] . According to this model, the absorption spectrum A( ) is described as a sum of the exciton peak absorption, X( ) and continuum band absorption Con( ): and 0 , , , , , and represent absolute exciton energy, exciton binding energy, exciton peak width, continuum edge width, asymmetric broadening of exciton peak and the step height of continuum edge, respectively.
As shown in Figure S12, the fitting results in the exciton binding energies of 300 ± 30 meV for 4ML NPLs and 320 ± 20 meV for 3ML NPLs, which a in agreement with reported values ranging from 260 to 350 meV 7,8 .
With estimated exciton binding energies, , and exciton energy, , obtained from absorption spectra, the energy shift in VB and CB from 4ML to 3ML NPLs was calculated according to: 9

S16
where and ℎ represent the effective mass for electron and hole of CsPbBr3 NPLs. Since similar effective mass has been reported for electron and hole of CsPbBr3 NPLs regardless of their thickness. 10 The CB and VB shift from 4ML to 3ML is estimated to be the same at 95 meV.

Estimation of the average number of molecules on each NPLs
It has been reported that at high photon energies the intrinsic absorption coefficient of CsPbBr3 nanocrystals is size-independent. 11 This means based on the absorbance of NPLs at high photon energies, such as at 335 nm, the total volume of NPLs in solution can be estimated. With average lateral size and thickness of NPLs obtained from TEM measurements, the total number of 4ML NPLs are estimated to be 15 times larger than that of 3ML NPLs. Since both NPLs lead to similar PLQY for each acceptor molecule (Figure 2c and 2f), it is reasonable to assume that the total amount of molecules attached are similar for both NPLs. Accordingly, the average number of molecules attached on 3ML NPLs should be 15 times larger than that of 4ML NPLs.

Kinetics fitting
Single-wavelength fitting for NPLs growth excited at 490 nm.
As the formation rate of the NPLs bleach directly represents the hole transfer rate, the dynamics at the wavelengths of the main exciton bleach for NPLs were fitted to extract the hole transfer rate. Specifically, the obtained dynamics are considered as a convolution of the real dynamics of the sample by the Instrument Response Function (IRF) of the setup. Hence, to extract the rates from the real dynamics, the TA data was fitted by an analytical form for the convolution of a function describing hole transfer by a function describing the IRF 12 .
The IRF function was described by a Gaussian function: The NPL growth was described by a step-function multiplied by an exponential ingrowth of a fraction of the signal: where is the Heaviside function and (1 − ) accounts for the initial signal at 0 due to coherent artifact ( Figure S7).
Global and Target analysis.
The two-dimensional TA data were analyzed by global and target analysis using Glotaran 13 .
With this method, the 2D data matrix, Ψ( , ), is modeled as a linear combination of n components given by the equation: Each component has its own spectrum, ( ), that following a certain concentration profile, In order to adequately fit the excited state kinetics of CsPbBr3 NPLs, a three-step sequential model was used (Figure S14). Based on this model, upon photoexcitation above the band gap of NPLs, the hot exciton relaxes to form the band-edge exciton with a cooling rate constant, . Subsequently, the short lifetime on the order of tens of picosecond is assigned to the shallow trapping, , followed by a longer lifetime of a few nanoseconds as the radiative recombination lifetime, . Fitting parameters are listed in Table S1.
Target analysis of excited state kinetics of NPLs+PDI/PMIDE systems excited at 380 nm To model the excited state kinetics of the hybrid systems, we assume each NPL is attached with a PDI/PMIDE as the concentration of acceptor molecules is much higher than the concentration of NPLs. In order to keep the kinetic model as simple as possible, a four-step sequential model was firstly used: Hot X → X → CS state → triplet state → GS. However, with this sequential model, the characteristics of PDI/PMIDE anion absorption was found in the decay-associated spectra of the triplet state. In order to obtain a better species-associated spectra of the triplet state, a brunched model (target analysis) was used ( Figure S14). In this model, a small fraction of CS state is allowed to decay directly to the ground state without forming the triplet states. The spectra of second and forth species in the NIR region are forced to be the same, in order to reduce the contamination of anion absorption in the triplet spectra.
Despite the difference in the global and target analysis models, the first (cooling of hot exciton) and second (formation of CS state) rate constants are very similar in both sequential and brunched models. Note that the second rate constant (k2) will not only be influenced by the charge transfer rate but also, to some extent, the fast trapping in the NPLs. In case of energy transfer followed by the ultrafast hole transfer, k2 should reflect the total contribution of both processes. Fitting parameters are listed in Table S2.